...once the rows reach the root in the execution plan tree, they are returned back to fetch call which sends them +back to the client (in case of a SELECT statement). In case of DML statements, the root rowsource function is the one which performs the required changes on the rows returned to the tree (update, delete).

...

Many operators also have options which affect how exactly the operator works. For example INDEX operator has options such as UNIQUE, RANGE SCAN, SKIP SCAN etc, which determine the way the INDEX operator internally works.

Starting from Oracle 11.1 it is possible to list all (non-hidden) rowsources available by querying X$XPLTON (eXPLain rowsource Operator Names).
The rowsource operator options are listed in X$XPLTOO (eXPLain rowsource Operator Options). Starting from Oracle 11.2, the same data can be queried from AWR views DBA_HIST_PLAN_OPERATION_NAME and DBA_HIST_PLAN_OPTION_NAME.